[Airhead Wrangler]

I wouldn't penetrate the wall of the tank if I were you. That seems like a losing proposition no matter how you seal it up again. I'd have someone weld or braze a blind threaded insert into the wall of the tank and screw them into that.

EDIT: Considering the difficulty involved in doing the above, the 'fake screw' and adhesive method mentioned earlier is what I would do.